Winter Severity and Wolf Predation on
a Formerly Wolf-free Elk Herd

Study Area

Yellowstone National Park is an 891,000-ha protected area primarily in northwestern
Wyoming, USA, with a variety of habitats from high alpine (3,800 m) to sagebrush
grasslands (1,500 m; Despain 1990, Cook 1993). The climate is characterized
by long, cold winters with substantial snow, and short, cool summers. Annual
precipitation varies from 26 to 205 cm and falls mostly as snow. Average monthly
temperatures range from -12°C to 13°C (Cook 1993).

The park is home to approximately 22,000 elk, 4,000 mule deer (Odocoileus
hemionus), 2,800 bison (Bison bison), 700 moose (Alces alces),
300 bighorn sheep (Ovis canadensis), 600 pronghorns (Antilocapra
americana), and scattered mountain goats (Oreamnus americanus;
Singer and Mack 1993). During our study, 48 wolves occurred in 9 packs during
1997, whereas 68 wolves occurred in 8 packs during 1998. About half of the
wolves wore radiocollars. In the northeastern corner of the park (Fig. 1),
where most of the elk spend late winter and spring, we studied the Druid Peak
pack (5 members in 1997, 8 in 1998), the Rose Creek pack (10 members in 1997,
15 in 1998), and the Leopold pack (5 in 1997, 9 in 1998). Wolf density in
this northern range was 25 wolves/1,000 km² in 1997 and 46/1,000 km²
in 1998.